The present invention relates to vessels used for high temperature, high pressure microwave-assisted reactions, including but not limited to digestion of materials in robust solvents.
In general, the term “digestion” refers to the process of analyzing the contents of a material by dissolving that material in a solvent that reduces the compounds that make up the material into their constituent elements or more basic compounds. In such form, the elements or compounds originally present in the material (the “analytes of interest”) can be identified more easily both as to their presence and their amounts. In many cases. however, the analytes of interest comprise only a small portion of the bulk of the material to be digested. As a result, the remaining unanalyzed portion of the material must be removed in order to free the analytes of interest for further analysis.
As one example, a soil sample can be analyzed for the presence of particular contaminating materials such as heavy metals by heating the sample in a strong acid that breaks down the bulk of the soil material (the matrix) and solvates the heavy metals making them available for further analysis. The resulting solution of elements can be diluted or otherwise prepared and then analyzed for content and quantity, for example using mass spectroscopy, atomic absorption spectroscopy, atomic emission spectroscopy, or other well-understood techniques.
Some materials will digest in acid at room temperature (i.e., about 20° C.). Other materials will digest when heated to moderate or somewhat elevated temperatures; e.g. 100-150° C. Other materials, however, will resist digestion until the temperature is raised to at least 200° C. and in some cases even higher.
Additionally, both the nature of digestion and in some cases the composition of the materials being tested result in chemical reactions that generate gases as part of the digestion process. These gases are commonly incidental side products of the breakdown of the matrix of the material. Conversion of the unanalyzed portion of the material to gaseous by-products can be seen as an important part of the digestion process—essentially freeing the analytes for further analysis. The solvents used to effect the digestion process are commonly liquids whose boiling points have a known relationship with temperature and pressure.
As dictated by the ideal gas law (and the more complex version of the gas laws), a gas that is heated to a higher temperature within the defined volume of such a sealed vessel will exert a correspondingly increased pressure against that vessel.
In pressurized digestion techniques the temperature of the process is elevated by carrying out the digestion in a sealed heated container. This allows the reaction to reach temperatures above the atmospheric boiling point of the digestion solvent. Increasing the temperature also increases the rate of the chemical reactions which accomplish the digestion. The digestion is thus more complete and faster as temperature is increased.
In microwave assisted digestion, in which the use of microwaves further accelerates the heating process, a sealed pressure vessel is used to contain the digestion reaction. Because metals tend to shield microwaves or cause sparking in a microwave field, microwave digestion is typically carried out in a microwave transparent vessel formed of an engineering polymer such as polyamide. At the temperatures commonly used for digestion, the pressure in the vessel is generated from two components. Vapor pressure generated by the digestion solvent(s) represents one component, and this component is predictable based upon the temperature of the solvent. The pressure of gaseous by-products generated during the digestion process represents the second component. Thus the amount of pressure in the vessel is related to both the boiling point of the solvent and also to the size and composition of the sample that is to be digested.
Because samples to be analyzed typically contain unknown amounts of material(s) that may form gaseous by products, the resulting amount of gas pressure is unpredictable.
Microwave transparent pressure vessels are commonly made from engineered plastics that can withstand relatively high pressures before failing. The nature of polymers and plastics is such, however, that if the vessel fails under pressure, it will tend to fail catastrophically.
In order to avoid such catastrophic failure, vessels for microwave digestion have been developed that include some means for pressure release. In some cases, the pressure release is provided by a small pathway leading from the interior to the exterior of the vessel with a small portion of the pathway blocked by a diaphragm that will fail at a predetermined pressure. When the pressure in such a vessel exceeds the predetermined limit, the diaphragm will burst and the gases will vent from the vessel without any catastrophic or near-catastrophic failure.
Commonly assigned U.S. Pat. Nos. 6,258,329; 5,520,886; 5,427,741; 5,369,034 and 5,230,865 are representative of the diaphragm type of pressure release system for vessels used in microwave assisted digestion and related reactions.
Accordingly, vessels have been developed in which the pressure release is temporary rather than complete and which allow the reaction to continue during and after the pressure release. Such vessels are designed to vent a small amount of gas when the pressure in the vessel exceeds predetermined limit and to re-seal themselves when the pressure drops below the predetermined limit. Examples include commonly assigned U.S. Pat. Nos. 6,927,371; 6,287,526; 6,136,276 and 6,124,582.
Such vessels commonly operate at 180-200° C. and cannot contain sufficient pressure to allow higher temperatures to be achieved.
If these vessels are sealed in a manner that attempts to contain gas pressures generated at temperatures above 200° C. (typically by over-tightening threaded fixtures), a higher proportion of these vessels will fail.
Such failures, of course, reduce efficiency by forcing experiments to be repeated. More importantly, when such vessels are heated above 200° C. and when they release the gases at such temperatures, the release tends to permanently distort the vessel even though catastrophic failure is avoided.
Because the vessels are formed of sophisticated engineering plastics, they tend to be relatively expensive. As a result, vessel failures result in the economic loss of the vessel in addition to the loss of the particular experiment and the loss of overall efficiency of the testing being carried out.
Although the capacity to carry out a digestion in sealed pressure-releasing vessels at temperatures up to 200° C. is valuable in many circumstances, there are a number of types of materials that will not digest even at such temperatures and that must be heated significantly above 200° C. before they will digest completely. If a composition fails to digest completely, the likelihood increases that elements will be mis-identified, identified in erroneous quantities, or remain completely unidentified.
For example, materials such as polymers, lubricating oils, high molecular weight compositions, compositions containing significant proportions of aromatic compounds, and refractory materials all need to be heated higher than 200° C. before they will digest properly. As an example, analyzing plastics in childrens' toy to make sure that it avoids containing undesirable (or in some cases prohibited) amounts of heavy metals or other materials requires such high-temperature digestion. The same is true for many lubricating and related oils which are widely present in a wide variety of industrial machinery as well as automobiles, trucks, trains and airplanes.
Digestion samples often contain very small amounts of the analytes of interest. The sample size which can be digested in any sealed vessel at a given temperature is thus limited by the safe operating pressure limit of the vessel. Maximizing sample size while maintaining a sufficient temperature for an effective digestion is an important aspect of the technique and increases the accuracy of the analysis
Therefore, a need exists for vessels suitable for microwave assisted chemistry that can withstand higher temperatures, can contain higher pressure, and can release pressure above a predetermined limit, but while avoiding or minimizing the loss of gases that may contain materials that need to be identified and quantified and while avoiding permanent or catastrophic failure of the vessel.